A community split among dolphins: the effect of social relationships on the membership of new communities.

Nishita M, Shirakihara M, Amano M - Sci Rep (2015)

Bottom Line:
Before the split, the males in the same community after the split more often associated with each other than they did with those in different community.These results indicate that the males of new community were socially different from the other males for a long time before the split, but the females might not have been different.The long-term social relationships among males could be maintained in the subsequent communities.

ABSTRACTLittle is known about community splitting among dolphins because such events are rare in dolphin populations. A case of a community split was confirmed in a population of Indo-Pacific bottlenose dolphins (Tursiops aduncus) in Japan, where a group of approximately 30 dolphins moved to a new habitat some 60 km from the original habitat. We examined the associations among the dolphins before the community split to determine whether the new community members were already socially different before the split, using 7-year identification data. Before the split, the males in the same community after the split more often associated with each other than they did with those in different community. In contrast, the association patterns among females and between sexes showed no relationships with their post-split community membership. These results indicate that the males of new community were socially different from the other males for a long time before the split, but the females might not have been different. Our findings suggest that at time of the community split, the factors determining the memberships of the subsequent communities are sex-linked. The long-term social relationships among males could be maintained in the subsequent communities.

f3: Dendrogram constructed from the average-linkage hierarchical cluster analysis for male dolphins between 1994–2000, using the HWI and HWIG values.The individuals are represented by an ID number with the letter N or S, each of which indicates the community class. The cophenetic correlation coefficient for the clustering using HWI is 0.82 and that using HWIG is 0.80. The cophenetic correlation coefficient values greater than approximately 0.8 indicate a good match of the dendrogram to the association matrix.

Mentions:
The association indices among males within the community class were higher than those between the community classes in both cases, using the HWIs and HWIGs as association indices (Mantel test, P < 0.00001, respectively). This result indicates that the males in the same community class were more likely to associate with each other than they were with those in the other community class. To examine the details of the association relationships among males, a dendrogram was constructed using average-linkage hierarchical cluster analysis for males, as shown in Fig. 333. In this dendrogram, most of the males in the same community class were clustered together, with the exceptions of NC male #220 when we used the HWI as the association index, and SC males #142, #94 when we used the HWIG as the association index (Fig. 3). In the clustering using the HWIG as the association index, the SC males were more tightly clustered than the NC males (Fig. 3). We compared the association indices among the NC males and those among SC males to determine whether the SC males exhibited stronger associations than the NC males (Fig. 4). The randomization test revealed that the HWIs among NC males were significantly higher than those among SC males (P < 0.0001). In contrast, the HWIGs among the SC males were significantly higher than those among the NC males (randomization test, P < 0.0001). The higher HWIs among the NC males than the SC males indicated higher gregariousness among the NC males than the SC males. Given their differences in gregariousness, the associations among SC males were stronger than those among the NC males.

f3: Dendrogram constructed from the average-linkage hierarchical cluster analysis for male dolphins between 1994–2000, using the HWI and HWIG values.The individuals are represented by an ID number with the letter N or S, each of which indicates the community class. The cophenetic correlation coefficient for the clustering using HWI is 0.82 and that using HWIG is 0.80. The cophenetic correlation coefficient values greater than approximately 0.8 indicate a good match of the dendrogram to the association matrix.

Mentions:
The association indices among males within the community class were higher than those between the community classes in both cases, using the HWIs and HWIGs as association indices (Mantel test, P < 0.00001, respectively). This result indicates that the males in the same community class were more likely to associate with each other than they were with those in the other community class. To examine the details of the association relationships among males, a dendrogram was constructed using average-linkage hierarchical cluster analysis for males, as shown in Fig. 333. In this dendrogram, most of the males in the same community class were clustered together, with the exceptions of NC male #220 when we used the HWI as the association index, and SC males #142, #94 when we used the HWIG as the association index (Fig. 3). In the clustering using the HWIG as the association index, the SC males were more tightly clustered than the NC males (Fig. 3). We compared the association indices among the NC males and those among SC males to determine whether the SC males exhibited stronger associations than the NC males (Fig. 4). The randomization test revealed that the HWIs among NC males were significantly higher than those among SC males (P < 0.0001). In contrast, the HWIGs among the SC males were significantly higher than those among the NC males (randomization test, P < 0.0001). The higher HWIs among the NC males than the SC males indicated higher gregariousness among the NC males than the SC males. Given their differences in gregariousness, the associations among SC males were stronger than those among the NC males.

Bottom Line:
Before the split, the males in the same community after the split more often associated with each other than they did with those in different community.These results indicate that the males of new community were socially different from the other males for a long time before the split, but the females might not have been different.The long-term social relationships among males could be maintained in the subsequent communities.

ABSTRACTLittle is known about community splitting among dolphins because such events are rare in dolphin populations. A case of a community split was confirmed in a population of Indo-Pacific bottlenose dolphins (Tursiops aduncus) in Japan, where a group of approximately 30 dolphins moved to a new habitat some 60 km from the original habitat. We examined the associations among the dolphins before the community split to determine whether the new community members were already socially different before the split, using 7-year identification data. Before the split, the males in the same community after the split more often associated with each other than they did with those in different community. In contrast, the association patterns among females and between sexes showed no relationships with their post-split community membership. These results indicate that the males of new community were socially different from the other males for a long time before the split, but the females might not have been different. Our findings suggest that at time of the community split, the factors determining the memberships of the subsequent communities are sex-linked. The long-term social relationships among males could be maintained in the subsequent communities.